Hearing assistance devices, such as hearing aids, include, but are not limited to, devices for use in the ear, in the ear canal, completely in the canal, and behind the ear. Such devices have been developed to ameliorate the effects of hearing losses in individuals. Hearing deficiencies can range from deafness to hearing losses where the individual has impairment responding to different frequencies of sound or to being able to differentiate sounds occurring simultaneously. The hearing assistance device in its most elementary form usually provides for auditory correction through the amplification and filtering of sound provided in the environment with the intent that the individual hears better than without the amplification.
In order for the individual to benefit from amplification and filtering, they must have residual hearing in the frequency regions where the amplification will occur. If they have lost all hearing in those regions, then amplification and filtering will not benefit the patient at those frequencies, and they will be unable to receive speech cues that occur in those frequency regions. Frequency translation processing recodes high-frequency sounds at lower frequencies where the individual's hearing loss is less severe, allowing them to receive auditory cues that cannot be made audible by amplification.
One way of enhancing hearing for a hearing impaired person was proposed by Hermansen, Fink, and Hartmann in 1993. “Hearing Aids for Profoundly Deaf People Based on a New Parametric Concept,” Hermansen, K.; Fink, F. K.; Hartmann, U; Hansen, V. M., Applications of Signal Processing to Audio and Acoustics, 1993. “Final Program and Paper Summaries,” 1993 IEEE Workshop on, Vol., Iss, 17-20 October 1993, pp. 89-92. They proposed that a vocal tract (formant) model be constructed by linear predictive analysis of the speech signal and decomposition of the prediction filter coefficients into formant parameters (frequency, magnitude, and bandwidth). A speech signal was synthesized by filtering the linear prediction residual with a vocal tract model that was modified so that any high frequency formants outside of the range of hearing of a hearing impaired person were transposed to lower frequencies at which they can be heard. They also suggested that formants in low-frequency regions may not be transposed. However, this approach is limited in the amount of transposition that can be performed without distorting the low frequency portion of the spectrum (e.g., containing the first two formants). Since the entire signal is represented by a formant model, and resynthesized from the modified (transposed) formant model, the entire signal may be considerably altered in the process, especially when large transposition factors are used for patients having severe hearing loss at mid and high frequencies. In such cases, even the part of the signal that was originally audible to the patient is significantly distorted by the transposition process.
In U.S. Pat. No. 5,571,299, Melanson presented an extension to the work of Hermansen et. al. in which the prediction filter is modified directly to warp the spectral envelope, thereby avoiding the computationally expensive process of converting the filter coefficients into formant parameters. Allpass filters are inserted between stages in a lattice implementation of the prediction filter, and the fractional-sample delays introduced by the allpass filters determine the nature of the warping that is applied to the spectral envelope. One drawback of this approach is that it does not provide direct and complete control over the shape of the warping function, or the relationship between input frequency and transposed output frequency. Only certain input-output frequency relationships are available using this method.
In U.S. Pat. No. 5,014,319, Leibman relates a frequency transposition hearing aid that classifies incoming sound according to frequency content, and selects an appropriate transposition factor on the basis of that classification. The transposition is implemented using a variable-rate playback mechanism (the sound is played back at a slower rate to transpose to lower frequencies) in conjunction with a selective discard algorithm to minimize loss of information while keeping latency low. This scheme was implemented in the AVR TranSonic™ and ImpaCt™ hearing aids. However, in at least one study, this variable-rate playback approach has been shown to lack effectiveness in increasing speech intelligibility. See, for example, “Preliminary results with the AVR ImpaCt Frequency-Transposing Hearing Aid,” McDermott, H. J.; Knight, M. R.; J. Am. Acad. Audiol., 2001 March; 12 (3); 121-7 11316049 (P, S, E, B), and “Improvements in Speech Perception with use of the AVR TranSonic Frequency-Transposing Hearing Aid,” McDermot, H. J.; Dorkos, V. P.; Dean, M. R.; Ching, T. Y.; J. Speech Lang. Hear. Res. 1999 December; 42(6):1323-35. Some disadvantages of this approach are that the entire spectrum of the signal is transposed, and that the pitch of the signal is, therefore, altered. To address this deficiency, this method uses a switching system that enables transposition when the spectrum is dominated by high-frequency energy, as during consonants. This switching system may introduce errors, especially in noisy or complex audio environments, and may disable transposition for some signals which could benefit from it.
In U.S. Patent Application Publication 2004 0264721 (issued as U.S. Pat. No. 7,248,711), Allegro et. al. relate a method for frequency transposition in a hearing aid in which a nonlinear frequency transposition function is applied to the spectrum. In contrast to Leibman, this algorithm does not involve any classification or switching, but instead transposes low frequencies weakly and linearly and high frequencies more strongly. One drawback of this method is that it may introduce distortion when transposing pitched signals having significant energy at high frequencies. Due to the nonlinear nature of the transposition function (the input-output frequency relationship), transposed harmonic structures become inharmonic. This artifact is especially noticeable when the inharmonic transposed signal overlaps the spectrum of the non-transposed harmonic structure at lower frequencies.
The Allegro algorithm is described as a frequency domain algorithm, and resynthesis may be performed using a vocoder-like algorithm, or by inverse Fourier transform. Frequency domain transposition algorithms (in which the transposition processing is applied to the Fourier transform of the input signal) are the most-often cited in the patent and scholarly literature (see for example Simpson et. al., 2005, and Turner and Hurtig, 1999, U.S. Pat. No. 6,577,739, U.S. Patent Application Publication 2004 0264721 (issued as U.S. Pat. No. 7,248,711) and PCT Patent Application WO 0075920). “Improvements in speech perception with an experimental nonlinear frequency compression hearing device,” Simpson, A.; Hersbach, A. A.; McDermott, H. J.; Int J Audiol. 2005 May; 44(5):281-92; and “Proportional frequency compression of speech for listeners with sensorineural hearing loss,” Turner, C. W.; Hurtig, R. R.; J Acoust Soc Am. 1999 August; 106(2):877-86. Not all of these method render transposed harmonic structure inharmonic, but they all share the drawback that the pitch of transposed harmonic signals are altered.
Kuk et. al. (2006) discuss a frequency transposition algorithm implemented in the Widex Inteo hearing aid, in which energy in the one-octave neighborhood of the highest-energy peak above a threshold frequency is transposed downward by one or two octaves (a factor of two or four) and mixed with the original unprocessed signal. “Linear Frequency Transposition: Extending the Audibility of High-Frequency Information,” Francis Kuk; Petri Korhonen; Heidi Peeters; Denise Keenan; Anders Jessen; and Henning Andersen; Hearing Review 2006 October. As in other frequency domain methods, one drawback of this approach is that high frequencies are transposed into lower frequencies, resulting in unnatural pitch transpositions of the sound. Additional artifacts are introduced when the harmonic structure of the transposed signal overlaps the spectrum of the non-transposed harmonic structure at lower frequencies.
Therefore, an improved system for improved intelligibility without a degradation in natural sound quality in hearing assistance devices is needed.